I think the common-sense definition that a lot people use, especially people not deeply invested in the movement, is something along the lines of "the source is available for everyone to examine and make modifications to".
It's not as specific as the OSI definition of course, and it doesn't say anything about redistribution and such, but that's okay and it doesn't really need to be IMO. Some vagueness is okay, and even with the OSI definition there is a huge difference between something like MIT and AGPL, both of whom are "open source".
A large majority of software developers and related professions use OSI's definition in my experience. Even the ones apathetic or antipathetic to the movement. The elements of OSI's definition have practical consequences for them.
People talk about permissive and copyleft licenses when the distinction matters. Covering both doesn't make open source vague.
Alternative definitions restrict the freedom to run the software at all frequently. Never mind modifying it.
It's not as specific as the OSI definition of course, and it doesn't say anything about redistribution and such, but that's okay and it doesn't really need to be IMO. Some vagueness is okay, and even with the OSI definition there is a huge difference between something like MIT and AGPL, both of whom are "open source".